X-ray exposure system



l J. M.` coNs'rABl.; ETAL 2,351,486'

June 13, 1944.

"X-RAY EXPOSURE SYSTEM meer if. V. .rre-fu' Patented June 13, 1944 X-RAY EXPOSURE SYSTEM James M. Constable, Baltimore, Md., and Roger W. Stamm, New York, N. Y., assignors to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsyl- Vania Application July 7, 1942, Serial No. 449,996

(Cl. Z50-95) 9 Claims.

This invention relates to X-ray apparatus and more particularly to protective systems for preventing X-ray tubes from damage due to overloads.

It is known to provide X-ray apparatus with current or voltage-responsive overload breakers for limiting the anode current or anode voltage. It has also been suggested to mechanically connect two rheostats for selecting the heating cur'- rent and anode current, respectively, with the pointer of a time scale'for indicating the maximum exposure period permissible with the chosen adjustments of heating and anode current.

These known devices are far from satisfactory as regards simplicity and reliability of operation and construction. Some of the known devices are rather complicated and exacting as to proper attendance. Others impose undue limitations on the adjustability of the operating condiitons of the X-ray tube, or are difficult to adapt to the correct ratingof the tube over a suiiicient range of anode voltages.

It is an object of the Present invention to provide an X-ray exposure system with an overload protection which avoids the drawbacks of the known devices of this' type. More in particular, the invention aims at providing an exposure system in which an overload protection is obtained by preventing the delivery of energy to the X-ray tube When the combined magnitudes of anode voltage, anode current and exposure time result in a tube load in excess of the tube rating.

Another object is to achieve the just-mentioned protection by electrical means of simple and reliable construction and operation which require no particularly skillful attendance.

It is also an object of the present invention to permit, in a protection system as aforementioned, a free choice of the magnitudes of anode current, anode voltage and exposure time Within a wide range of techniques.

.A further object is to provide a protective X-ray exposure system that lends itself'readily to a highly accurate adjustment or calibration of its protective members in accordance with the rating of the particular tube or type of tubes employed.

According to the present invention, an X-ray exposure system, containing a, timing device for adjusting the duration of each exposure and comprising circuit means for also selecting the anode current and anode voltage of the X-ray tube, is provided with a protective relay or the like control device for preventing, when operated, the delivery of energy to the tube. The protective relay is controlled by two voltages, the magnitude of one corresponding to that of the voltage (kilovoltage) in the anode circuit of the tube, while the magnitude of the other control voltage is a function of the selected exposure time, i. e., the duration of the energy supply. The two control voltages are caused to act on the relay so that the latter responds to a combined effect and prevents the excitation of the tube when this effect goes beyond a limit value determined by the rating of the tube.

ln another aspect of the present invention and representing a preferred embodiment, the energyresponsive control voltage is supplied by a voltage source, such as a transformer, autotransformer or potentiometer, connected or associated with the excitation circuit of the X-ray tube so that this control voltage is proportional to the X-ray kilovoltage. The other control voltage, in this embodiment, issupplied by a potentiometric or the like device which is mechanically coupled with the timing device so that the voltage magnitude depends on the selected exposure period.

According to another feature of the invention, at least one of the two voltage sources for controlling the protective relay of the X-ray exposure system is calibrated in accordance with a non-linear law so that the resultant control eiect on the relay is substantially in accordance with the usual non-linear tube rating. An especially simple apparatus of this typel is obtained if the dependency of the energy-responsive control voltage is straight-proportional to the excitation voltage-of the tube while the timeresponsive'control voltage follows a non-linear dependency.

These and other objects and features of the present invention will be understood from the following description of the embodiment illus-l trated inthe drawing, in which Fig. 1 is a circuit diagram of an X-ray exposure system according to the present invention, while the other figures, all relating to the embodiment of Fig. 1, are of explanatory nature and show in Fig. 2 e rating chart of an X-ray tube, in Figs. 3 to 7 voltage diagrams referring to the `two control voltages and their combined eifect, and in Figs. 8 to l0 rating charts serving to determine the constants of the protective relay circuit.

Referring to the exposure system shown in Fig. 1, il) is the main supply line connected to a main switch or overload breaker Ilv. A control transformer l 2 has its primary I3 connected with the line I over an anode current selector in the form of a dial switch I4 whose contact bank is connected with corresponding taps of the primary. The secondary I5 of the control transformer is tapped in both end regions and connected with two dial or selector switches I6 and I1, respectively, for adjusting the anode voltage (kilovoltage) of the X-ray tube. Selector I6 has large voltage steps for major voltage adjustments such, for example, as in terms of tens, and selector I1 small steps for minor adjustments in terms of units. The two selectors I6 and I1 are series-connected in a circuit I8 with the primary I9 of a high-tension transformer 20 whose secondary 2| serves to excite the X-ray tube 22 over a rectifier set 23.

Series-arranged in circuit I8 are also the contacts 24 of a relay 25 whose winding 26 is connected to a manually adjustable timing device in order to be energized, for closing the contacts 24, during periods of selected duration in accordance with the setting of the timer. The selected period is indicated by a pointer 28 on the time scale of the timer. The timing device 21 is connected with the line I0 over a starting switch 29 and the contacts 30 of a protective relay 3I. When actuating the switch 29, the timing device passes current from the line IU to the winding of the timer relay 26 during each operating period of the timer and -provided the contacts 30 of the protective relay are closed.

The winding 32 ofv the protective relay 3I lies in a control circuit 33 in series with the output connections of a potentiometer comprising a resistor 34r and a slide contact 35. The potentiometer is fed from the line I0 through a transformer 36. The control circuit 33 is connected with the two selector switches I6 and I1. Thus the winding 32 of the protective relay is energized by a result voltage composed of the voltage drop Vi derived from the potentiometer, and another voltage Vkv which corresponds to that across the primary I9 and hence to the anode voltage (kilovoltage) of the X-ray tube 22. In the illustrated system, the two voltages Vt and Vkv have the same direction so that the protective relay is controlled in dependence upon the sum of these voltages.

The slide contact 35 of the potentiometer is connected with the timer mechanism or pointer 28,V for instance, by mechanical means as indicated at 31.

When preparing an exposure, the selector switch I4 is set in accordance with the desired anode current (M. A.), and each selector I6 and For a proper operation of the exposure system, it is of course necessary that the dependency of the resultant voltage magnitude in control circuit 33 on the settings of the voltagesupplying circuit elements be so related to the load characteristic of the X-ray tube 22 that the operation of the protective relay corresponds with sufficient accuracy to the rating of the tube throughout the available ranges of anode voltage and exposure time. Therefore, a certain selection of the constants of the control circuit 33, in particular a suitable calibration of its voltagesupplying elements, is required. In this respect, an embodiment of the illustrated type has the advantage that only one circuit element, namely, the potentiometric resistor 34, requires a nonlinear calibration, i. e., a non-linear dependency of its output voltage upon the adjustment of the timing device, in order to adapt the system to the non-linear rating of the customary X-ray tubes. Such a non-linear operation of thepotentiometer is obtained, for instance, by a corresponding non-linear graduation of its resistance steps.

The operation of the system and the abovedescribed adaptation of the circuit constants to the tube rating will be more fully understood from the following. taken in conjunction with Figs. 2 through 10 of the drawing.

Fig. 2 represents, in type, the cold rating chart of an X-ray tube for a given value of anode current, the K. V. values of the anode voltage being plotted on the ordinate, while the abscissa represents time values (t) indicating the duration of the energy delivery to the tube, i. e., the exposure periods. Exposures corresponding to points within the shaded area are permissible. Exposure conditions represented by points beyond curve I are overloads and must be avoided to prevent damage. i

It is required that the control circuit of an exposure system according to the invention represent a tube rating of the above-mentioned type by the.l resultant of two component voltages. By way of illustration let it be considered that one of these component voltages (Viiv), according toy Fig. 3, is proportional to the K. V. value, and the other component voltage (Vt), according to Fig. 4, is proportional to the exposure time t. Then, if for every point on curve I in Fig. 2,

I1 in. accordance with the desired anode voltage.

In addition, the timer 21 is set for a contemplated exposure period. If after these settings are chosen the two voltages Vkv and Vi add up to a total below the critical operating voltage of the protective relay 3|, the relay contacts 3l] remain closed. Consequently, when the switch 29 is closed by the operator, the timer 21 is energized and causes the timer relay 25 to close the excitation circuit I8 of the tube for the selected exposure period. If, however, the settings of the selectors and timer combine so as to produce a resultant control voltage Vzw-i-Vf, which is higher than the critical voltage of the protective relay 3|', the relay coil 32 receives sufficient voltage to open the contacts 30. When the starter switch 25 is actuated, the timer 21 remains ineffective of voltages Vkv and Vi are added and the sum plotted against t as abscissa, the curve P in the diagram of Fig. 5 is obtained. The curve P represents a new boundary between permissible and non-permissible exposures in terms of a voltage scale, and indicates the maximum permissible kilovoltage (e) that maybe used: within a given time value (t).

It will be seen from Fig. 5 that if a relay were to be used under the above assumptions to discriminate between the two voltage regions, the relay would have to respond to different critical voltage values (P), depending upon the selected exposure periods. In order to avoid the difficulties of such a requirement, the following modicatio-n of the voltage characteristic is appropriate.

Referring to the tube rating chart of Fig. 2, curve I can be transformed into a straight line by distorting the time (t) scale. In other words, the curve I can be represented as a straight line by applying a correspondingly non-linear gradation of the time values. Performing now the above-described substitution by voltages and leaving the component voltage'Vkv straight-proportional to the kilovoltage as represented by Fig. 3, the second component voltage Vt will be changed to a characteristic as shown, in principle, by Fig'. 6. In this gure, the t 'scale is again uniform, while the voltage curve follows a non-linear course in dependence upon the non-linearity of the tuber rating. Adding now Vkv and Vt according to Figs. 3 and 6, respectively, a new resultant voltage curve is obtained which is of linear characteristic and extends in parallel to the t axis as represented by curve P1 in Fig. 7. P1 is again the boundary between permissible and non-permissible exposures, i. e., a locus of points (on I, see Fig. 2) which represent maximum combinations of KV and t. Thus, for KV=KVT1 and t=T1 (Figs. 3 and 6) Ev=VkvT1+VT1=p (0n P1). For KV'=KV1'1 and t T1, '20:17 and the exposure is permissible. When KV---KVT1 and t T1, EU p so that the exposure is not permissible. Since the critical voltage is now constant (p) for all available exposure periods, some contacting device or relay can easily be employed and can be made to operate at the critical p value with sufficiently close limits.

Reverting now to the exposure system exemplied in Fig. 1, it will be clear from the foregoing that the operation of the protective relay can in fact be b-rought in conformity with the rating of the X-ray tube by supplying one voltage (Vkv) proportional to the kilovoltage, and another voltage (Vt) in a given non-linear dependency on the exposure time.

For determining the characteristic of this dependency and hence the required calibration of the timer-controlled potentiometer, the following method can be employed to advantage.

A general rating chart of an X-ray tube has the appearance of Fig. 8 where the parameters a, b, c and d denote different values of anode current, the values decreasing in that order. For any such rating chart which has been determined .from the physical relations and experimental data governing the vloading of the tube, aA transformation can be effected by distorting the t scale so that the parametersa, b, c, d become straight ilnes, al1 intersecting at KV=0 and -at some Value of t determined by the design of the tube. The resulting modified chart is exemplified by Fig. 9. t denotes a distortion of the original tscale (hereinafter referred to as t only).

This transformation, together with the method I explained previously with Yreference to Figs. 2 through 7, leads to the construction of a diagram, represented 4by Fig. l0, from which the circuit design of the protective device may be read off with ease. In the diagram. of Fig. 10, the value Vx at which the protective relay will open is chosen as the maximum voltage for both time (t) and KV voltage ordinates (plotted to the same scale). Curves a, b, c and dare drawn in which interceptv on the KV-volts ordinate proportional to the values of anode current. Referenceto Fig. 8 permits determining for any value of KV the corresponding valu-e of t. The relay operating voltage Vx, less the KV-voltage, will give the time-voltage for the associated time value (t). In this way, the characteristic of the time-voltage is determined, as a non-linear curve 1- over a uniform time scale (t). Curve 1- represents the dependency of the component voltage-Vt on the exposure time' as adjusted by the timing device.

lAccording to a preferred embodiment of the invention, the movable control member of the Vt-control element, exemplied by the slide contact 35 in Fig. 1, is coupled with the timer to operate in synchronism therewith. In such an embodiment the above-mentioned dependency is brought about by gradatlng the resistance 34 inl accordance with the curve f determined by the aforedescribed method. However, the gradation of the potentiometer resistor 34 may be uniform if the connection with the timer, indicated by 36, involves a transmission such as a cam gear Whose law of transmission corresponds to curve r.

Other modifications, such as the use of a protective relay of a type different from the illustrated contactor, the insertion of a transformer or the like intermediate circuit element between the circuits I8 and 33, or the substitution of variable inductances, transformers or auto-transformers, etc. for the illustrated potentiometer, will be obvious to those skilled in the art and hence are not shown in the drawing. It is essential for any such modification that two voltages, one dependent on the energy delivery to the tube and the other on-v the exposure time, be supplied to a protective de-j vice so that their combined effect prevents the energy delivery when its magnitude is beyond a limit value determined by the rating of the X-ray tube to prevent the tube from overloading.

We claim: l

l. An X-ray exposure system, comprising an X-ray tube, circuit means connected with said tube for the delivery of energy to said tube, a timing device for controlling said circuit means in accordance with a selected exposure time, a protective relay connected with said circuit means for preventing energization of said X-ray tube by overload conditions, means for supplying a voltage depending upon the magnitude of the energy supplied lby said circuit means to said X-ray tube, and means for supplying another voltage depending upon the selected exposure time, both said Voltage supply means being connected with said relay for actuating the latter when tho combination 0f said energy magnitude and said exposure time corresponds to a load exceeding the rating of said tube. l

2. An X-ray exposure system, comprising an X-ray tube, a circuit arrangement connected with said tube for the delivery of energy to said tube, a timing device for controlling said circuit arrangement in accordance with a selected exposure time, a-protective relay connected with said circuit arrangement for preventing energization of said X-ray tube upon given load conditions, circuit means connected with said arrangement for supplying a voltage proportional to the energy supplied by said arrangement to said X-ray tube, and additional circuit means connected with sai-d timing device for supplying another voltage depending upon the selected-exposure time, both said circuit means being con-l nected with said relay to control the latter in accordance with the combined effect of said two voltages and cause operation of said relay to prevent said circuit arrangement from energizing said X-ray tube when said effect corresponds to a tube load exceeding the rating of said tube. A 3. An X-ray exposure system, comprising an X-ray tube having a non-linear rating, a circuit arrangement connected with said tube for the delivery of energy to said tube, a timing device for controlling said circuit arrangement in accordance with a selected exposure time, a protective relay connected with said circuit arrangement for preventing energization of said X-ray tube upon given load conditions, circuit means connected with said arrangement for supplying a voltage in linear proportion to the Lenergy supplied to said X-ray tube by said circuit arrangement and additional circuit means connected with said timing device for producing another voltage in non-linear relation to said exposure time depending upon said non-linear tube rating, both said circuit means being connected with said relay to control the latter in accordance with the combined effect of said two voltages and to cause operation of said relay to prevent energization of said X-ray tube when said effect corresponds to a tube load exceeding said rating.

l4. An X-ray exposure system, comprising an X-ray tube, a circuit arrangement connected with said tube for the delivery of energy to said tube, a timing device for controlling said circuit arrangement in accordance with a selected exposure time, a protective relay connected with said circuit arrangement for preventing energize.- tion of said X-ray tube upon given load conditions, two voltage supply devices connected with said relay for providing two component voltages, respectively, to control said relay in acordance with a resultant effect caused by said two voltages, one of said devices being connected with said circuit arrangement to derive therefrom a component voltage proportional to the energy supplied to said X-ray tube, the other of said devices having a voltage source, a pot-entiometer connected with said source, and a movable contact member connected with said timing device to move in synchronism with and proportionally to the operation of the later, said potentiometer being gradated'in accordance with a non-linear rate depending upon the rating of said tibe, whereby said relay operates to pre- Y vent 'energization of said X-ray tube when said resultant effect of said two voltages corresponds to a load exceeding said tube rating.

5. An X-ray exposure system, comprising an X-ray tube, a circuit arrangement connected with said tube for the delivery of anode current to said tube, said arrangement containing variable circuit means for selecting the voltage of said anode current, a timing device for controlling said circuit arrangement in accordance with a selected exposure time, a protective relay connected withsaid circuit arrangement for preventing energization of said X-ray tube under given exposure conditions, control means connecting said variable circuit means with said relay to produce a control eiTect on the latter in proportion to said selected anode voltage, further control means disposed between said timing device and said relay to produce another control effect on the latter in a given relation to said selected exposure time, whereby said relay operates to prevent energization of said X-ray tube when the combination of said two control eiects exceeds a value corresponding to a tube load exceeding the rating of said tube.

6,'An X-ray exposure system, comprising an X-ray tube, a circuit arrangement connected with said tube for the delivery of anode current to said tube, said arrangement containing variable circuit means for selecting the voltage of said anode current, a timing device for controlling said circuit arrangement in accordance with a selected exposure time, a protective relay having relay contacts connected with said circuit arrangement, and a winding for causing said relay, When actuated, to prevent energization of said X-ray tube, a control circuit connecting said Winding with said circuit arrangement to supply said winding with a voltage component having a 75 magnitude depending on that of said anode voltage, and vo-ltage supply means controlled by said timing device and connected in said control circuit for supplying said winding with another voltage component depending upon said exposure time in order to actuate said relay when the sum of said voltage components exceeds a value corresponding to a tube load exceeding the rating of said tube.

7. An X-ray exposure system, comprising an X-ray tube, a transformer device for supplying said tube with anode current, said device having primary selector` means for adjusting the intensity of said current and secondary selector means for adjusting the voltage of said current, a circuit disposed between said secondary selector means and said tube, a timing device having a movable member for setting the exposure time of said X-ray tube, a voltage source having a movable part for varying the magnitude of its output voltage, said part being connected with said movable member of said timing device to move in a given relation to said member to cause said output voltage to be dependent upon the selected exposure time, and a relay arranged for controlling said current supply to said tube and connected with both said circuit. and said voltage source in order to be controlled by a resultant voltage in accordance with a combined effect dependent on the voltage of said anode current and on said output voltage to prevent said current supply when said combined eiect corresponds to a tube load exceeding the rating of said tube.

8. An X-ray exposure system, comprising an X-ray tube having a non-linear tube rating, a transformer device for the delivery of anode energy to said tube, said device having primary selector means for adjusting the anode current and secondary means for adjusting the anode voltage of said energy, a circuit disposed between said secondary selector means and said tube, a timing device having a movable member for ad-V justing the duration of said energy delivery, a`

variable voltage source connected with ysaid timing device to cause its output voltage to vary in a given non-linear relation to the adjustments of said movable member depending upon said non-linear tube rating, and a relay arranged to control said current supply to said tube and connected with both said circuit and said voltage source and controllable by a resultant voltage in accordance with a combined effect dependent on the voltage of said anode current and on said output voltage to prevent said current supply when said combined eiect corresponds to a tube load exceeding the rating of said tube.

9. An X-ray exposure system, comprising an X-ray tube having a non-linear tube rating, a transformer device for the delivery of anode energy to said tube, said device having primary selector means for adjusting the anode current and secondary means for adjusting the anode voltage of said energy, a circuit disposed between said secondary selector vmeans and said tube, a timing device having a movable member for adjusting the duration of said energy delivery, a potentiometric voltage source having a resistor and a movable member for providing a variable output voltage, said movable potentiometer member being connected with said movable member of said timing device to move in linear dependency upon the adjustment of the latter member, said resistance being gradated in accordance with a non-linear law depending upon said non-linear tube rating, a relay having contacts arranged in said circuit and a Winding for 4causing said relay when actuated, to prevent said energy delivery to said tube, a control circuit connecting said Winding with both said circuit and said potentiometric voltage source for controlling said Winding by the sum of tWo Voltages derived from said circuit and said voltage source respectively, whereby said relay is actuated to prevent said delivery when said sum corresponds t0 a tube load exceeding the tube rating.

JAMES M. CONSTABLE. ROGER W. STAMM. 

